This invention relates to DC/AC power converters, and more particularly to bi-directional DC/AC power converters.
Prior art bi-directional DC/AC power converters can be divided into three categories including: line frequency transformer, non-sinusoidal converters; line frequency transformer, sinusoidal converters; and high frequency transformer, sinusoidal converters.
Line frequency transformer non-sinusoidal converters typically employ an H-bridge of switching elements connected to a primary winding of a transformer designed to operate at AC line frequencies and phase control switching elements connected to a secondary winding of the transformer. Switching of the switching elements forming the H-bridge is controlled to produce a quasi-squarewave at the primary winding of the transformer, and this quasi square wave is stepped up by the turns ratio of the transformer. The secondary winding of the transformer produces an output waveform which is commonly called a quasi sinewave or a modified sinewave. Regulation of the root mean square value of the output voltage of the converter is achieved by varying the duty cycle of the waveform. The phase controlled switch is employed to regulate charging current, in the charging mode. By advancing or retarding the phase angle relative to the zero crossing of the AC voltage, the current can be increased or decreased. Power converters of this type have disadvantages in that the output voltage in inverter mode is non-sinusoidal and is only regulated for its Root Mean Square (RMS) value. Some loads are sensitive to non-sinusoidal waveforms and only operate well when the applied voltage is sinusoidal. Others are sensitive to the peak voltage of the AC waveform and require regulation of the peak voltage close to the peak voltage of the sinewave voltage. In addition, with this type of power converter a pulsating current is drawn from the battery, in the inverter mode, as there is no or little internal energy storage within the converter. In addition, power factor in the charger mode is low and current distortion is high. In addition, charging current is pulsating and, finally, the line current transformer is relatively large and heavy, limiting the applications of the converter.
With power converters of the line frequency transformer, sinusoidal type, it is common to find a multi-transformer configuration having secondary windings wired in series and primary windings connected to an H-bridge of switches. By controlling the switching sequence of the switches, a stepped sinusoidal voltage is produced on the secondary windings. Switching H-bridges may also be controlled to convert an AC voltage applied to the transformer secondary winding to produce a DC voltage for battery charging. U.S. Pat. No. 5,373,433 to Thomas discloses this approach.
Line frequency transformer, sinusoidal inverter/chargers also include a line frequency transformer with a multi-tapped secondary winding. The transformer also has a primary winding which is driven with a quasi-squarewave as described above. Bi-directional switches selectively connect the taps of the secondary winding to the output to produce a roughly stepped approximation of a sinusoidal output voltage. This approach is disclosed in U.S. Pat. No. 5,155,672 to Brown.
Uninterruptible power supply circuits also normally fall into the line frequency transformer sinusoidal inverter charger category as they involve an H-bridge controlled by sinusoidal pulse width modulation to produce a sinusoidal line frequency voltage at primary terminals of a line frequency transformer. The line frequency transformer provides a step up or step down in voltage and galvanic isolation between the DC port and AC port. This circuit, however, lacks internal energy storage and therefore, produces pulsating currents at the DC port in both the inverter and charger modes. In addition, such devices are large and heavy because they require one or more low frequency transformers.
Power converters of the high frequency transformer, sinusoidal type include bi-directional DC to high frequency AC converter stages which are connected to a low voltage winding of a high frequency transformer. A power converter of this type is described in U.S. Pat. No. 4,742,441 to Akerson. The high frequency transformer provides voltage step up and step down and galvanic isolation. A high voltage winding of the transformer is typically connected to a high frequency AC to low frequency AC cycloinverter, the output of which is used to source power to an AC load or receive power therefrom. Use of the cycloinverter requires the use of bi-directional switches which, with present technology, must be constructed as composite assemblies of uni-directional switches. In addition, the switches in the DC to high frequency conversion stage and the high frequency AC to low frequency AC conversion stage must be precisely synchronized when switching to avoid destroying the switching elements. This requires complex control circuits.